The present invention generally relates to ink cartridge printing systems, and more particularly to an ink cartridge having a control unit designed to prevent the reverse flow of ink and air into the cartridge reservoir.
The development of sophisticated ink jet printing systems has created a corresponding need for improved ink delivery cartridges. To satisfy this need, ink cartridges have been manufactured which include an ink reservoir in fluid communication with an orifice plate and resistor assembly. Activation of the resistor assembly generates heat which thermally excites the ink and expels it from the cartridge. This delivery procedure is known in the art, and described in the Hewlett-Packard Journal, May 1985, Vol. 36, No. 5. In addition, an exemplary cartridge using thermal ink excitation is illustrated in U.S. Pat. No. 4,500,895.
In the system described above, ink within the reservoir is normally maintained at a negative pressure. Ink is drawn out of the reservoir by capillary action which occurs when the ink is excited and expelled from the orifice plate. However, this type of system often experiences problems associated with the reverse flow of air and ink into the reservoir. This is caused by the negative pressure within the reservoir which draws air into the cartridge and pulls ink away from the orifice plate. When ink is displaced from the orifice plate, the printing process is interrupted, causing what is commonly known as a "deprime." In addition, the introduction of air into the cartridge and reservoir causes the ink therein to bubble or foam, thereby reducing the operating efficiency of the cartridge. Furthermore, any air in the reservoir will expand when ambient temperatures are increased. This expansion can force ink out of the reservoir and cartridge when not in use, causing uncontrolled leakage.
A variety of methods have been attempted to eliminate problems associated with the reverse flow of ink and air into ink cartridges. For example, a system exists in which a small screen is positioned in the cartridge between the ink reservoir and orifice plate. The surface tension of the ink and small mesh size of the screen minimize the reverse flow of air and ink into the reservoir. However, this technique is not entirely effective, and frequently causes the generation of air bubbles in the cartridge when the vacuum from the reservoir pulls air through the screen. As a result, the operating efficiency of the cartridge is reduced. A need therefore exists for an ink cartridge system having means for effectively controlling the reverse flow of air and ink into the system. The present invention satisfies this need in a manner not heretofore known in the art, as described below.